Use a testbed analysis. You need to place the CPU in another board and test or another CPU in that board.

I have seen this happen before and more so when such voltages are applied to AMD Processors and more so to Semprons as they are at the bottom end of the CPU testing grounds. As has been already mentioned, this is an event that has been seen before. I would say that since the issue is not the PSU, then I would next look to the issue being the Sempron.